1. Field of the Invention
The present invention relates to optical fiber filters, and particularly to improved fiber Bragg grating filters.
2. The Prior Art
Fiber Bragg grating filters are known in the art. Fiber Bragg grating filters are becoming widely used because of the increased use of fiber optical systems. For example, fiber Bragg grating filters are used to filter out noise or unwanted wavelengths of light in optical distribution systems.
FIG. 1 is a diagram showing the operation and measurement of a prior-art fiber Bragg grating filter. FIG. 1 shows an optical fiber 1 incorporating a Bragg grating portion 2. As is known by those of ordinary skill in the art, Bragg grating portion 2 is formed by applying energy such as laser energy to an optical fiber. The resulting deformations in the optical fiber increases the refractive index of the fiber periodically as shown in FIG. 1.
The effects of fiber Bragg grating filters on applied light is demonstrated in FIG. 1 by applying the output A of a wide bandwidth light source to optical fiber 1. When the applied light A encounters the Bragg grating portion 2, some light will pass through the Bragg grating portion 2 and be transmitted as light T, and some of the applied light A will be reflected by the Bragg grating portion 2 and will travel back to the source as light R.
Typically, fiber Bragg grating filters are designed to pass or reflect a desired wavelength, or band of wavelengths, of light. FIG. 2 is a graph typical of such a design. FIG. 2 shows a graph where the vertical axis represents the percentage of reflected light R reflected back to the source, as was shown in FIG. 1 by reflected light R. The horizontal axis represents the wavelength of light, and is scaled in nanometers (nm). As can be seen from FIG. 2, a typical fiber Bragg grating filter will form a band-reject filter, rejecting a predetermined band of light having a central wavelength of xcexc. One example of a prior art fiber Bragg grating filter is designed to have a xcexc in the range of 1,500 nm to 1,650 nm. Fiber Bragg grating filters may also be defined to have a reject band, shown as RB in FIG. 2. Reject band RB is defined as the range where the response of the fiber Bragg grating filter is greater than 3 dB. As is known by those of ordinary skill in the art, the inverse of the plot shown in FIG. 2 will show the transmitted light T rather than the reflected light R, and the inverse of the region defined as the reject band RB will be shown as the pass band.
As mentioned above, often fiber Bragg grating filters are utilized in optical distribution systems and thus may be deployed in any number of locations, such as buried underground. Therefore, fiber Bragg grating filters must be able to function in a wide variety of hostile environments, such as drastic seasonal temperature fluctuations. One problem associated with prior-art fiber Bragg grating filters is their sensitivity to environmental temperature fluctuations. The materials that fiber Bragg grating filters are constructed from tend to expand or contract depending upon the ambient temperature. For example, as temperature increases, the refractive index of the fiber increases due to the positive temperature dependence of the refractive index of the fiber, which is approximately 5.6xc3x9710xe2x88x926/xc2x0 C. The temperature coefficient of the central wavelength of pass-band is about 0.01 nm/xc2x0 C. Thus, xcexc as shown in FIG. 2 will tend to shift with the ambient temperature.
However, the refractive index of the fiber is proportionately dependent on its internal stress. Thus, relieving tension on the fiber as temperature increases can decrease the refractive index to compensate for the temperature induced wavelength shift of all Bragg grating filters. Tension can be released as temperature increases by thermal expansion of a substrate having a negative thermal expansion coefficient to which the fiber is attached, or by thermal induced bending of a bi-metal plate. The prior art has successfully used this technique to provide thermal-compensated fiber Bragg grating filters.
For example, in U.S. Pat. No. 5,042,898 to Morey et al., a portion of a length of an optical fiber incorporating a grating filter is fixedly mounted between two compensating members formed from a material having selected thermal expansion coefficients. The first compensating member is preferably fabricated from a material having a high thermal expansion coefficient. The second compensating member may be formed from a material having a low thermal expansion coefficient, as for example Invar or Fused silica. The positions of the two compensating members are then adjusted with respect to one another such that the fiber portion incorporating the grating filter may be placed under a preloaded tensile stress condition.
During the subsequent use of the device following the tension preloading of the fiber portion containing the Bragg filter grating region, the effect of differential thermal expansion coefficients between the materials of the compensating members decreases the tension applied to the fiber portion with a temperature increase and increases the tension applied to the fiber portion on a temperature decrease, thus compensating for the wavelength change of the filter with the changing temperature. The rate of relieving tension can be selected by choosing materials with appropriate thermal expansion coefficients for the compensating members and by adjusting the geometry in order to hold the wavelength of the Bragg filter constant. In one disclosed embodiment, the material of one of the compensating members has a larger thermal coefficient of expansion than the material of the other compensating member. As a consequence, the tensile stress of the fiber portion containing the filter or grating region will be decreased as temperature increases and increased as temperature decreases.
Another prior-art thermal-compensated Bragg grating filter employs a portion of a length of an optical fiber incorporating a grating filter rigidly connected in tension between the ends of a c-shaped frame formed from a first material. The c-shaped frame is mounted at its center portion to a strip of a second material having a thermal coefficient of expansion different from that of the first material to form a bimetallic strip. The bi-metallic strip bends with temperature change as is known in the art, thus deforming the center section of the c-shaped frame comprising one of its elements and changing the tension exerted on the portion of the optical fiber incorporating a grating filter disposed between the two ends of the frame.
Yet another prior-art thermal-compensated Bragg grating filter is disclosed in U.S. Pat. No. 5,841,920 and employs an optical fiber incorporating a Bragg grating filter in which the central wavelength varies in opposite directions with temperature and with an axial strain applied to the grating. A top surface of a compensating member is affixed to a portion of the fiber that includes the grating and a tension adjusting member is connected to the compensating member. The tension adjusting member and the compensating member are formed of materials selected so that as the temperature of the device decreases, the tension adjusting member contracts more than the compensating member so as to control the deformation of the compensating member and thereby impose an axial strain on the grating. The controlled application of tensile stress to the optical fiber in the region of the grating causes changes in the central wavelength of the pass band of the filter to compensate for central wavelength variations resulting from changes in the grating temperature.
The prior-art thermal compensated Bragg grating filters are suitable for their intended purpose. However, they all have one or more certain drawbacks, including long lengths or other dimensions. Further, the prior-art thermal compensated Bragg grating filters are relatively complex and expensive to manufacture. Furthermore, prior art devices that apply axial tension to the optical fiber tend to have a high failure rate due to the fact that over time the applied tension will cause failures in the optical fiber or the means used to attached the optical fiber.
It is therefore an object of the present invention to provide a thermal-compensated Bragg grating filter that overcomes the problems of the prior art.
It is another object of the present invention to provide a thermal-compensated Bragg grating filter that is easier and less expensive to manufacture than those of the prior art.
Yet another object of the present invention to provide a thermal-compensated Bragg grating filter that is smaller in size than those of the prior art.
Yet another object of the present invention to provide a thermal-compensated Bragg grating filter that is more reliable than those of the prior art.
According to first aspect of the invention, a thermal compensated Bragg grating filter comprises a compensating member and a torsion adjusting member rotatedly mounted to the compensating member. An optical fiber having a Bragg grating therein is laid adjacent to a torsion member, and the two are then rigidly affixed between the torsion adjusting member and the compensating member. The torsion adjusting member may then be rotated, twisting the optical fiber around the torsion member, and thus applying torsion to the optical fiber containing the Bragg grating.
According to a second aspect of the invention, a thermal compensated Bragg grating filter comprises a compensating member having a longitudinal axis and a torsion adjusting member rotatedly mounted to the compensating member. An optical fiber containing a Bragg grating filter therein is rigidly affixed between the torsion adjusting member and the compensating member in an axis other than the compensating member""s longitudinal axis. The torsion adjusting member may then be rotated to apply torsion to the optical fiber.
According to a third aspect of the invention, a thermal compensated Bragg grating filter comprises an optical fiber containing a Bragg grating filter therein and a compensating member having a desired thermal coefficient of expansion. The optical fiber is bent through a curve and affixed to the compensating member such that torsion is applied to the Bragg grating region of the optical fiber.
Other aspects of the invention provide for bending the optical fiber through a curve, for twisting the optical fiber, and for twisting the optical fiber around another member, and then affixing the optical fiber to a compensating member having a selected thermal coefficient of expansion.
The present invention provides an arrangement constructed from one component for introducing in an optical fiber a strain that compensates for temperature induced changes in wavelength reflected by grating. The thermal-compensated Bragg grating filter of the present invention has fewer parts than and is simpler and less expensive to manufacture than prior-art thermal-compensated Bragg grating filters. The thermal-compensated Bragg grating filter of the present invention is also smaller in length than prior-art thermal- compensated Bragg grating filters. The thermal compensated Bragg grating filter of the present invention also has substantially more thermal linearity than those of the prior art, and is more reliable.